Microelectronic devices are typically manufactured from a semiconductor material, such as silicon, germanium or gallium/arsenide. The semiconductor material is fashioned into a die, a generally rectangular structure having circuitry on one surface. Along the periphery of the surface are input/output pads for electrical interconnection to external components. The device is brittle and requires protection from moisture and mechanical shock. This protection is provided by a package which encapsulates the device. The package further contains an electrically conductive means, such as a leadframe, to transport signals between the device and external circuitry.
Package components are made from ceramics, plastics and metals. Ceramic packages are characterized by excellent reliability. Ceramics are brittle, expensive and poor conductors of heat. During operation, the device generates heat. If not removed, the device will overheat reducing operating life.
Plastic packages are not brittle and are relatively inexpensive to manufacture. The thermal performance of a plastic package may be improved by molding a metal heat spreader into the the body of the package. Plastic packages are susceptible to moisture permeation which may lead to corrosion damage to the device circuitry and to the bond wires.
Metal packages have better thermal conductivity and lower cost compared to ceramic packages. Metal packages have better hermeticity and reliability than plastic packages. For these reasons, metal packages are preferred for housing high power semiconductor devices which generate significant quantities of heat during operation.
A typical metal package is disclosed in U.S. Pat. No. 4,796,083 to Cherukuri et al. The package has metal or metal alloy base and cover components. Disposed between the base and the cover is a leadframe. A sealing glass bonds the leadframe to both. Alternatively, the sealing glass may be replaced with a polymer adhesive as disclosed in U.S. Pat. No. 4,888,449 to Crane et al. Both glass and adhesively sealed metal electronic packages frequently employ a centrally positioned die attach pad. The semiconductor device is bonded to the pad. The pad is then bonded to the package base with either a conductive means such as a solder or a dielectric means such as a sealing glass or adhesive.
The metal package may then have up to four parallel conductive planes separated by dielectric layers, the base, cover, leadframe and die attach pad. When a magnetic field passes through the structure, mutual inductance may result. Inductance is the generation of an electromotive force in one circuit by a change of current in an adjoining circuit. Each conductive plane of the metal package behaves as a circuit. A current induced in one component will influence the performance of the other components.
The magnetic field is frequently in the form of electromagnetic interference. EMI may originate external to the package in the form of radio waves, static electricity or other generated magnetic fields. The interference may also be internally generated by voltage changes in the device during operation. EMI can result in voltage potential deviations across the electronic device leading to soft errors and device malfunction. When mutual inductances between the leadframe and the other components becomes large, logic transition times are increased significantly by the time constants associated with the inductances. Both switching speed and noise characteristics are affected.
The internally produced EMI is capable of generating interference with external electronic devices and also must be shielded.
A second advantage of the invention is that the grounded base and cover are parallel planes separated from the leadframe by a low dielectric polymer adhesive, i.e. the epoxy seal. The base and cover form a ground plane. The ground plane reduces mutual capacitance between adjacent leads of the leadframe. This reduces the coupling of radiated EMI from one lead to another.
To maximize the performance of a metal package, it is desirable to shield the components and encased device from electromagnetic interference. Shielding of the semiconductor device has been accomplished by grounding the device to the base of the package. The use of a conductive die attach material such as a solder grounds the device. Extending a bond wire from an input/output pad on the face of the device to the package base is also used.
While suitable for grounding the electronic device to the package base, the leadframe and cover components are still free floating conductive planes and mutual inductances and the associated problems are still present. Extending a bond wire from the device to the package base requires additional processing steps and coating the base with a material suitable for wire bonding. Likewise, soldering requires coating the base with a material which is wet by the solder.